1. Field of the Invention
This invention relates to an apparatus and method for detecting a quickly or sharply varying signal against a background of a more slowly changing ambient signal.
2. Description of the Prior Art
Detection of an event in a signal is of interest in a number of different applications in different fields. For example, there are various systems for locating and identifying projectile motion and impact based on signals from sensors and transducers, such as microphones and vibration sensors. One example is described in copending U.S. application Ser. No. 208,673 entitled "Apparatus and Method for Determining Projectile Impact Locations, now U.S. Pat. No. 4,898,388 to Beard et al. The signals could easily be transmitted as electromagnetic radiation and picked up by antenna. As a further example, objects may be detected by means of their interference with and effect on radiation, as in radar and photoelectric cell devices. It may be helpful in order to detect the event in the signal and avoid false triggers, to screen ambient signals, referred to as "noise". For example, in the case of sound signals, the noise might correspond to wind, overhead airplanes, or nearby cars.
In U.S. Patent to Beard et al, there is illustrated and described an apparatus for determining and analyzing the impact location of golf balls. Microphones near that location pick up signals in response to the impact. Those signals are processed to identify the relative times of detecting the impact at various microphones and, by triangulation calculations, to determine the impact location. Beard notes that "it is desirable to screen out of the signals to be processed . . . randomly or slowly fluctuating background noise, which for present purposes is any signal which has a rise time significantly less than that of a ball impact sound, such as a passing automobile, wind, etc." However, the signal corresponding to the impact might be weak relative to the signal corresponding to the ambient background noise. Beard discloses broadly the use of a comparator having a predetermined and/or variable threshold for making an ongoing comparison of signals to separate detection of a sharply rising signal from detection of a more slowly rising signal and discard the latter. The instant invention is directed to an improved technique for making such comparisons for use in conjunction with systems of the type described by the Beard et al patent, as well as for use with detecting other types of events in other types of backgrounds.
The following patents are hereby referenced as being typical of the known prior art insofar as they disclose methods for screening out noise and locating and identifying objects:
______________________________________ Patent No. Inventor ______________________________________ 4,006,907 Russell H. Heffley, Jr. 4,305,142 Barry R. Springer 4,409,473 Naofumi Furuta et ano. WO 87/05706 (GB) Dennis Brian McCarthy et al. ______________________________________
Heffley discloses a transducer responsive to impact directly on a target zone. Springer discloses sensors responsive to the impact of ballistic projectiles travelling at supersonic speed. McCarthy discloses transducers responsive to shock waves created by supersonic projectiles. Furuta discloses photoelectric cells responsive to light beams reflected by or blocked by objects. All of these devices screen background noise by responding only to signals with amplitude greater than that of a predetermined or preset threshold. For example, in Furuta, that threshold is determined by the properties of a photoelectric cell, a relay, and a resistor. Such calibration is used in devices where the event signal is much stronger than the background noise signal. However, such a method is not particularly effective in systems, such as Beard et al, in which the event signal is weak relative to the ambient background noise signal.
Techniques for comparing the potential or current of one electric signal with another, reference signal, are well-known in the electric arts. Relays, Zener diodes, vacuum tubes and transistors may each be used to make such comparisons. The typical switching regulator used in power supplies has such a comparator, in which a time varying signal is compared to a constant reference voltage or current. See U.S. Pat. No. 3,975,672 issued to Eugene Lachocki; U.S. Pat. No. 4,195,335 issued to David Murakami; U.S. Pat. No. 3,641,267 issued to Eugenio Cavalarri; U.S. Pat. No. 4,074,325 to Sakai et al; and Furuta supra. As discussed above, such comparisons are not particularly effective in systems in which the event signal is weak relative to the ambient background noise signal.
Accordingly, it is a general object of this invention to detect a signal against a background signal in a manner in which constitutes an improvement over the prior art discussed above.
It is a further object of this invention to improve the accuracy of such detection when the event signal is quickly or sharply varying and the background signal is more slowly changing, by using a dynamic comparison based on the signals' different frequency characteristics.